Every board root keys once at its store — the hover path stops walking the filesystem

`DragLocality.isSameBoard` resolved symlinks on both URLs, which stats every
path component. It ran five-plus times per `dropUpdated` and again per lane per
body evaluation through `renderedCards` → `hiddenMembers` — order of 50–100
stat calls per mouse-move, and worse on iCloud-backed paths. A pickup stall
sample had it on ~47 of 943 stacks.

`BoardRootKey` mints that canonical spelling once, where the root is, and every
locality comparison downstream is an `==` on two strings. The drag carriers hold
keys rather than URLs, so re-deriving under the cursor is no longer expressible.
`rootURL` keeps the user's spelling — the folder name is the display-name
fallback, and canonicalizing there would visibly rename a board opened through
a pin. The key follows the folder through `relocate(to:)`: a key frozen at open
would collide with a new board opened at the vacated path, and two boards
comparing as one is a cross-board drag silently behaving as a move.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/UI/Board/CardFaceView.swift

@@ -720,7 +720,7 @@ struct CardFaceView: View {
     /// ▸ The trash), and the trash column's own delegate clears the file highlight rather than
     /// proposing one — so this is a second, structural statement of the same rule.
     private var isFileHovered: Bool {
-        role.container == .board && drops.session.fileAttachTarget(onBoardRooted: store.rootURL) == card.id
+        role.container == .board && drops.session.fileAttachTarget(onBoardRooted: store.rootKey) == card.id
     }
 
     /// **Board-only** — "everything edit-shaped is disabled on trash selections … Rename"